The present invention relates to the field of velocity or flow sensors. Fluid and gas velocity sensors that are commercially available are commonly of the single hot wire, hot film, or thermistor type and are mounted on the end of a probe which is inserted into the fluid stream. A detailed description of the operation and construction of the hot wire and hot film anemometers is given, for example, by E. Nelson, "Hot Wire and Hot Film Anemometry," Sensors, Sept. 1984, pp. 17-22. In principle, the thin wire or filament is heated to a given temperature by passing a current through the element. As the fluid velocity changes, the magnitude of heat transfer from the element to the fluid changes through convection and conduction which changes the temperature of the sensing element. The temperature change and associated change in electrical resistance caused by the cooling effect of the fluid flow is a measure of the fluid velocity. For maximum sensitivity, the product of temperature coefficient of resistance (TCR) and resistivity of the hot wire element must be large. A number of wire materials are adequate in this respect and are readily available in fine wire form. These include platinum, tungsten, nickel, and an alloy consisting of 80% platinum and 20% iridium. The TCR times resistivity products for these materials are 320, 250, 480, and 240.times.10.sup.-10 ohm-cm/.degree.C., respectively. (H. P. Grant and R. E. Kronaner, "Fundamentals of Hot Wire Anemometry," Symposium on Measurement in Unsteady Flow, American Society of Mechanical Eng., p. 44 (1962)). Other considerations for choosing the wire material include impact strength and gas temperature range, ease of mounting and welding, ease of cleaning, and specific factors such as freedom from catalytic surface properties.
In recent years, for many applications, the basic hot wire anemometer has been replaced by a hot thin film anemometer with film thickness being approximately 1000 Angstroms. Advantages of the thin film anemometer over the hot wire anemometer are improved frequency response, lower end loss of heat resulting in shorter sensing length, more flexible configuration (i.e., flat, wedge, conical), and less susceptibility to fouling. A thin quartz (SiO.sub.2) coating on the surface resists accumulation of foreign material. One major drawback of the thin film anemometer flow sensor is that it is relatively large in size and must be manufactured individually (which makes costs expensive). Several attempts to improve the thin film anemometer flow sensor have been made. These improvements have centered around the micromachining techniques used in integrated circuit manufacturing. A typical example is given by Higashi et al., U.S. Pat. No. 4,501,144. Higashi teaches a sensor which includes a pair of thin film resistors located on a thermally isolated bridge across a well that has been micromachined in a monocrystalline silicon substrate. Equi-distant between the resistors is a thin film heating element that, under operation, is heated to a temperature of approximately 200.degree. C. via I.sup.2 R heating. At zero fluid flow, the two resistor elements are heated to the same temperature and thus exhibit the same resistance. Under flow conditions, the upstream element is cooled by the incoming fluid to a temperature which is proportional to the fluid velocity. The downstream element is cooled by a lesser degree since the fluid has been heated by the upstream sensing element and by the heating element during its travel. Thus, the resistance change of the upstream element is larger than that compared to the downstream element. The difference of the two resistor readings, which can be converted to a voltage signal via on-chip or external circuitry, is a measure of flow velocity. By using batch processing techniques, several hundred of these sensors can be processed on a single silicon wafer. After dicing the wafers into appropriate chips, the individual sensing elements are then packaged and wire bonded to make the appropriate connections to the external circuitry. While Higashi offers improved performance in the area of reduced power consumption, the thin diaphragm structure is susceptible to fouling and breakage in hostile environments. And, while measuring the temperature difference of the two sensors eliminates the effects of thermal aging, the distance of the sensors from the heater reduces thermal conductance.
Huijsing et al (J. H. Huijsing, J. P. Schuddemat, and W. Verhoef, "Monolithic Integrated Direction-Sensitive Flow Sensor," IEEE Transactions on Electron Devices, Vol. ED-29, No. 1, pp. 133-136, January, 1982) disclose a flow sensor comprising a silicon chip with an identical diffused transistor temperature sensing element embedded near each upstream and downstream edge of the chip and a centrally located diffused transistor heater element to raise the chip to as much as 45 degrees Centigrade above the air stream temperature. The upstream sensing element is cooled slightly more than the downstream sensing element under air flow conditions, and the temperature difference between the two sensor element transistors results in an electrical current difference between them, which when converted to a voltage difference is a measure of the air flow. The sensor elements must be located on opposite sides of the chip to achieve an appreciable temperature difference between them, and even so, in the air flow range up to 1000 feet per minute, the temperature differences are small, range from 0 to under 0.2 degrees Centigrade.
Van Putten et al (A. F. P. Van Putten and S. Middlehoek, "Integrated Silicon Anomometer," Electronics Letters, Vol. 10, No. 21 October, 1974, pp. 425-426) disclose a silicon chip with an identical diffused resistor element embedded on each of four opposite sides of a chip. All resistor elements are self heated, thus raising the chip and its support substantially above the ambient air stream temperature. The resistors are operated in an electrical double bridge circuit. Under zero air flow, all elements are at the same temperature, and the double bridge is electrically balanced. Under air flow conditions, the upstream and downstream elements which are normal to the flow are cooled more than the side elements which are parallel to the flow. This temperature difference unbalances the electrical double bridge to give a measure of the air flow velocity.
The foregoing attempts improve the state of the art in some respects, but remain deficient with respect to many characteristics desired such as ruggedness. Therefore, it is an object of the present invention to provide a flow sensor having improved ruggedness to hostile environments.
It is another object of the present invention to provide a flow sensor having low power consumption and good thermal conductance.
It is yet another object of the present invention to provide a flow sensor that can be easily manufactured using conventional micromachining techniques.
It is another object of the present invention to provide a flow sensor whose active sensing elements are located on a thermally isolated substrate which minimizes energy consumption during operation.
These and other objects and advantages of the invention, as well as the details of the illustrative embodiments, will be more fully understood from the following description and the drawings.